Umbraco-CMS/src/Umbraco.Core/VersionExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Semver;

namespace Umbraco.Core
{
    internal static class VersionExtensions
    {
        public static Version GetVersion(this SemVersion semVersion, int maxParts = 4)
        {
            int build = 0;
            int.TryParse(semVersion.Build, out build);

            if (maxParts >= 4)
            {
                return new Version(semVersion.Major, semVersion.Minor, semVersion.Patch, build);    
            }
            if (maxParts == 3)
            {
                return new Version(semVersion.Major, semVersion.Minor, semVersion.Patch);
            }

            return new Version(semVersion.Major, semVersion.Minor);
        }

        public static Version SubtractRevision(this Version version)
        {
            var parts = new List<int>(new[] {version.Major, version.Minor, version.Build, version.Revision});
            
            //remove all prefixed zero parts
            while (parts[0] <= 0)
            {
                parts.RemoveAt(0);
                if (parts.Count == 0) break;
            }

            for (int index = 0; index < parts.Count; index++)
            {
                var part = parts[index];
                if (part <= 0)
                {
                    parts.RemoveAt(index);
                    index++;
                }
                else
                {
                    //break when there isn't a zero part
                    break;
                }
            }

            if (parts.Count == 0) throw new InvalidOperationException("Cannot subtract a revision from a zero version");

            var lastNonZero = parts.FindLastIndex(i => i > 0);

            //subtract 1 from the last non-zero
            parts[lastNonZero] = parts[lastNonZero] - 1;

            //the last non zero is actually the revision so we can just return 
            if (lastNonZero == (parts.Count -1))
            {
                return FromList(parts);
            }
            
            //the last non zero isn't the revision so the remaining zero's need to be replaced with int.max
            for (var i = lastNonZero + 1; i < parts.Count; i++)
            {
                parts[i] = int.MaxValue;
            }

            return FromList(parts);
        }

        private static Version FromList(IList<int> parts)
        {
            while (parts.Count < 4)
            {
                parts.Insert(0, 0);
            }
            return new Version(parts[0], parts[1], parts[2], parts[3]);
        }
    }
}